Phased array antenna systems are frequently used in satellites, for example to provide communication between a satellite and a ground station on the earth. As the satellite goes through its orbit, there are portions of the orbit in which the satellite antenna system can communicate with the ground station, and portions of the orbit in which the curvature of the earth prevents such communication. Consequently, the antenna system is operated during portions of the orbit, and is not operated during other portions of the orbit.
State of the art designs for phased array antenna systems include the use of miniature microwave integrated circuits (MMICs), positioned in close proximity to the antenna elements. The close proximity of the MMICs and antenna elements helps to reduce phase delays in radio frequency signals within the antenna system. On the other hand, the MMIC components generate a substantial amount of heat. If not properly cooled, this can interfere with antenna operation. In particular, temperature gradients across the antenna array must be minimized, because significant temperature gradients introduce phase errors into electrical radio frequency signals within the antenna system, which in turn leads to errors and/or inaccuracies during antenna operation.
In this regard, the maximum allowable temperature gradient across the array decreases as the operating speed of the array is increased. For example, when the phased array is operating at a frequency of about 5 GHz, the maximum allowable temperature gradient across the array is about 20° C. In contrast, when the array is operating at a frequency of about 80 GHz, the maximum allowable temperature gradient across the array is only about 1.3° C. If the maximum temperature gradient across the array cannot be kept within the appropriate limit, then it is necessary to provide additional circuitry in the antenna system, in order to effect dynamic phase error compensation control. Of course, this additional compensation circuitry increases the complexity, cost and weight of the phased array. Thus, it is desirable to have a cooling apparatus which is both efficient and effective.
Providing such a cooling apparatus in the specific environment of a satellite involves some relative stringent design criteria. The cooling apparatus must be relatively compact, and light in weight. Further, it must be highly reliable, and relatively maintenance free. One prior approach was to provide a cooling apparatus which included a housing with a chamber therein, the chamber containing both a phase change material and a thermally conductive porous material. Although such a cooling apparatus has been generally adequate for its intended purpose, it has not been satisfactory in all respects.
More specifically, it is not always possible to achieve cooling of a level sufficient to keep temperature gradients across the surface of an antenna system within a desired limit, particularly as operating frequencies increase. As a result, and as mentioned above, it can be necessary to provide special additional circuitry in the antenna system in order to effect dynamic phase compensation, thereby increasing the complexity, cost and weight of the antenna system. Further, with respect to antenna systems that are either large and/or high-power systems, the known cooling apparatus is not always capable of providing sufficiently efficient cooling to maintain junctions of the electrical devices at temperatures which are low enough to assure accurate and reliable operation. One result is a reduction in the accuracy and/or reliability of the overall antenna system.